The present invention relates to anisotropic etching of nitride layers. More particularly, the invention relates to an anisotropic etching process in which the etching gas uses three components, namely, CH3F, CO2 and O2.
Conventional dry etching processes for etching silicon nitride (Si3N4) films formed on silicon oxide (SiO2) films use a variety of etchant gases. The choice of etchant gas is somewhat limited, however, due to the miniturization of semiconductor devices which contain extremely thin nitride layers. Because these nitride layers have thicknesses on the order of 102-103 xc3x85, selectivity and control of etch rates are of great importance to maintaining control of etching processes.
In microelectronic fabrication processes, etching is the selective removal of sections or regions of material from either a silicon substrate or from other thin films on the substrate surface. Etching proceeds in all directions at the same rate in an isotropic etching process. Typically, mask layers are used to protect those regions of material which the fabricator wishes to maintain on the substrate or thin film. Generally, however, mask layers cannot exactly form the pattern desired. This inability results because, as the etching proceeds, some removal occurs in undesired places. FIG. 1 shows such an occurrence.
Referring now to the drawing, wherein like reference numerals refer to like elements throughout, FIG. 1 illustrates the results of an isotropic etching process in which some material has been removed in directions other than just the z-directionxe2x80x94despite the presence of a mask layer 100. Mask layer 100 was placed upon an insulator film layer 110 which is on top of substrate 120. During the etching process, however, mask layer 100 did not prevent some of insulator film layer 110 from being removed in areas 130 other than directly below the mask layer 100. Similarly, some parts of insulator film layer 110 remained in areas where etching was supposed to remove it.
FIG. 2 illustrates an anisotropic etching process, in which the insulator film layer 110 has been removed in all but exactly where mask layer 100 was placed. Thus, both the mask layer 100 and the insulator film layer 110 have exactly the same dimensions in the x-y plane. In such a case, where etching occurs in only one direction (here, only in the z-direction), the etching process is said to be completely anisotropic. Because the completely anisotropic process as depicted in FIG. 2 is only an ideal state (completely single dimensional etching is not achievable), anisotropic processes are those which achieve nearly xe2x80x9ccompletely anisotropicxe2x80x9d results.
Current typical anisotropic processing techniques for thin nitride layers (e.g., spacer or liner levels of 100-1,000 xc3x85) include the use of chlorinated-type gases (such as HBr/Cl2) to provide selectivity to underlying thin oxide layers. Although very high etch rates and high selectivity may be achieved, these processes offer no selectivity to underlying silicon layers (especially doped silicon layers, such as those used in Direct Random Access Memory (DRAM) processing). In particular, in many applications, such silicon or doped silicon underlying layers are rapidly eroded by chlorine in the event of oxide punch-through.
In an attempt to overcome the above problems, technologies requiring a very thin gate oxide (less than 100 xc3x85) have used a fluorine-based chemistry (e.g., CHF3 and CO2) to provide some measure of selectivity to silicon. Fluorine-based processes offer much improved selectivity to underlying silicon. Unfortunately, however, fluorine-based chemistries offer much lower nitride:oxide selectivity, typically 3:1 or less compared with 8:1 or better for the chlorine process. Fluorine-based chemistries are also generally associated with low etch rates.
Because of low uniformity of these slow, selective processes, punch-through of the oxide is a serious concern. An alternative process, using a hydrofluorocarbon (e.g., CH3F and O2) having a low fluorine:carbon (F:C) ratio, has been used to enhance the selectivity of the fluorinated processes. Although more selective (approximately 6:1 ratio of nitride:oxide) than the fluorinated process, the etch rate of this process is much lower, at approximately 180 xc3x85minxe2x88x921. Because of its low etch rate, the time to conduct the process will be substantially increased from the chlorinated process (by a factor of about 3).
Therefore, there remains a need for a process which accelerates the etch rate, improves the selectivity of current fluorine-based processes, and offers sufficient tunability/control for optimization for both thick and thin nitride layers. Accordingly, one object of the present invention is to provide a highly selective nitride:oxide anisotropic etch process for etching the nitride layer on top of an oxide layer.
To meet this and other needs, and in view of its purposes, the present invention provides a highly selective etching process for a nitride:oxide combination upon a substrate. The process comprises the combined use of a hydrogen-rich fluorohydrocarbon (e.g., CH3F or CH2F2), a strong oxidant (O2), and a carbon source (CO2 or CO). It is preferred that the following amounts of each source be used for optimal performance: 7%-35% CH3F; 1%-35% O2; and 30%-92% CO2 by volume.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.